Your logic makes no sense.
It makes absolutely perfect sense. You have only touched on the melee combat ability, and not on classes dedicated to other options other than combat. A wizard / cleric hybrid, for instance, is still going to be utter crap, even if you make things cheaper. Having spell slots grow isn't as large of a deal as not having the spell to go along with it. You can get a lot of mileage out of Fireball and Spirit Guardians, but to get those spells, we're looking at around character level 10. Even with a reduced cost, that's still going to be a fairly long point when others are getting their level 5 spells; a wizard of one level lower is summoning elementals and Planar Binding them, while a cleric has finally gotten Raise Dead. And you can have a fireball-throwing cleric if you just go light cleric; you're even doing good evoker-style damage thanks to Potent Spellcasting. And that is including one level difference.
"Cheaper" may let the characters level up faster, but that does come with its own complications as well. If you go more than one level difference, we start talking about causing some dissastisfaction with higher HD and Proficency bonuses as well.
We talked about spellcasters, because its fairly easy to understand, but now lets talk about experts. A lot of people just focus on combat, so that social and exploration skills do take a minor. In games like that, such skills and abilties really aren't important, and can be ignored. In other games, where they take a greater focus, not having these skills do hurt. There's more going on than having just a high dice roll bonus for tracking, infiltration, social-fu, and lore. Access to the abilities slightly faster than normal multi-classing do not make your goal of hybridization any closer in reach.
Claiming that making multiclassing cheaper drags expertise classes through the mud is akin to saying that reducing the prices in the above makes certain prices dearer.
Hon, its not even close to it. You're comparing apples and peas.
5e multiclass rules are not meant to create hybrid characters. Hells. It wasn't meant to work that way in 3.x. That was what certain Prestige Classes were for - you may have needed to multiclass to get into it as a "cost" but the ultimate end result was that, without those special classes, you could not do a good hybrid that functioned in the game by any meaningful mechanical metric.
Attempts to make it cheaper is not going to change that fact. I don't care if you make buying a lump of garbage cheaper by a pound. Its still trash that's not worth buying in the first place. All your plan would do is make dipping even more attractive, but still leave hybridization out of reach. The complete opposite of your intent, if I'm reading things right.
The simple fact of the matter is that multi-classing is not designed to do hybridization, so your attempts are going to meet with failure unless you redo the entire system at a deeper level than just "cheaper."